Abstract Decay accelerating factor (DAF, CD55) is a glycophosphatidylinositol (GPI)-linked, cell surface-expressed protein with a known primary function as an intrinsic (on the cell surface on which it is expressed) regulator of complement cascade activation. We among others discovered that DAF crucially modulates T cell immune responses by locally regulating complement activation. We also showed that during cognate T cell/APC interactions surface DAF expression is rapidly (within hours) yet transiently (24-48h) downregulated. We hypothesize that this transient downregulation crucially permits local complement activation to drive induction of effector T cells (Teff) while simultaneously inhibiting induction, function and stability of regulatory T cells (Treg). While DAF deficient animals have been produced and exhibit enhanced T cell immune responses, formal testing of this hypothesis requires the ability to constitutively enforce stable DAF expression on immune cell surfaces in vivo. Our hypothesis predicts that this enforced DAF expression will inhibit APC activation and limit induction of Teff immunity and simultaneously will permit and facilitate Treg induction/stability, together dampening protective anti-viral T cell immunity while favoring immunological tolerance directed toward allo- or autoantigens. If our working model is correct, the findings will be conceptually innovative and significant in that they would support a novel mechanism that exerts control over T cell immunity in vivo. The proposed work will result in production of a mouse with an inducible, stable surface expressed form of DAF, thereby adding an essential missing tool to the toolbox of biological reagents available for studying links between adaptive immunity and complement activation. We propose to test this hypothesis in this R21 application through 2 aims: 1) To modulate T cell immunity by enforced DAF expression in hematopoietic cells and 2) To produce and analyze T cell immune responses in an inducible, conditional, DAF transgenic mouse. The findings derived from this ?high-risk high-reward? R21 application are likely to provide new insight into the impact of DAF expression (rather than its absence) on in vivo T cell immune responses, an issue that has not been addressed and represents a gap in our current knowledge. If our working model is correct, the findings would provide the basis for novel therapeutic strategies to increase DAF expression in efforts to induce tolerogenic responses that prevent and/or treat autoimmune disease, transplant rejection and graft vs. host disease. In addition, at the completion of this R21 grant we will have produced novel biological tools including inducible DAF transgenic mice that will permit us to test DAF-dependent tolerogenic strategies in autoimmune and transplant models, determine whether and how DAF impacts induction of nave and/or memory T cell activation and function in each situation, and provide insight into mechanisms underlying physiological DAF downregulation.